Surface plasmon resonance imaging measurements of ultrathin organic films

Thiolene-based microfluidic flow cells for surface plasmon resonance imaging

Thiolene-based microfluidic devices have been coupled with surface plasmon resonance imaging (SPRI) to provide an integrated platform to study interfacial interactions in both aqueous and organic solutions. In this work, we develop a photolithographic method that interfaces commercially available thiolene resin to gold and glass substrates to generate microfluidic channels with excellent adhesion that leave the underlying sensor surface free from contamination and readily available for surface modification through self-assembly. These devices can sustain high flow rates and have excellent solvent compatibility even with several organic solvents. To demonstrate the versatility of these devices, we have conducted nanomolar detection of streptavidin-biotin interactions using in situ SPRI.

Patterned resonance plasmonic microarrays for high-performance SPR imaging

We report a novel optical platform based on SPR generation and confinement inside a defined three-dimensional microwell geometry that leads to background resonance-free SPR images. The array shows an exceptionally high signal-to-noise ratio (S/N > 80) for imaging analysis and subnanometric thickness resolution. An angular sensitivity of 1°/0.01 RIU has been achieved and the signal to background ratio (S/B) improves to 20, 1 order of magnitude higher than that of the best literature results. The design proves effective for probing-supported lipid membrane arrays in real time with a thickness resolution of 0.24 nm and allows for imaging analysis of microfluidic circuits where resonant spots are separated by only one pixel (∼7 μm). The high image quality and unique chip geometry open up new avenues for array screening and biomicrofluidics using SPRi detection.

Surface plasmon resonance phase imaging measurements of patterned monolayers and DNA adsorption onto microarrays

The optical technique of surface plasmon resonance phase imaging (SPR-PI) is implemented in a linear microarray format for real-time measurements of surface bioaffinity adsorption processes. SPR-PI measures the phase shift of p-polarized light incident at the SPR angle reflected from a gold thin film in an ATR Kretschmann geometry by creating an interference fringe image on the interface with a polarizer-quartz wedge depolarizer combination. The position of the fringe pattern in this image changes upon the adsorption of biomolecules to the gold thin film. By using a linear array of 500 μm biosensor element lines that are perpendicular to the interference fringe image, multiple bioaffinity adsorption measurements can be performed in real time. Two experiments were performed to characterize the sensitivity of the SPR-PI measurement technique: First, a ten line pattern of a self-assembled monolayer of 11-mercaptoundecamine (MUAM) was created via photopatterning to verify that multiple phase shifts could be measured simultaneously. A phase shift difference (Δφ) of Δφ = 182.08 ± 0.03° was observed for the 1.8 nm MUAM monolayer; this value agrees with the phase shift difference calculated from a combination of Fresnel equations and Jones matrices for the depolarizer. In a second demonstration experiment, the feasibility of SPR-PI for in situ bioaffinity adsorption measurements was confirmed by detecting the hybridization and adsorption of single stranded DNA (ssDNA) onto a six-component DNA line microarray patterned monolayer. Adsorption of a full DNA monolayer produced a phase shift difference of Δφ = 28.80 ± 0.03° at the SPR angle of incidence and the adsorption of the ssDNA was monitored in real time with the SPR-PI. These initial results suggest that SPR-PI should have a detection limit roughly 100 times lower than traditional intensity-based SPR imaging measurements.

Fabrication and anti-fouling properties of photochemically and thermally immobilized poly(ethylene oxide) and low molecular weight poly(ethylene glycol) thin films

Poly(ethylene oxide) (PEO) and low molecular weight poly(ethylene glycol) (PEG) were covalently immobilized on silicon wafers and gold films by way of the CH insertion reaction of perfluorophenyl azides (PFPAs) by either photolysis or thermolysis. The immobilization does not require chemical derivatization of PEO or PEG, and polymers of different molecular weights were successfully attached to the substrate to give uniform films. Microarrays were also generated by printing polymer solutions on PFPA-functionalized wafer or Au slides followed by light activation. For low molecular weight PEG, the immobilization was highly dependent on the quality of the film deposited on the substrate. While the spin-coated and printed PEG showed poor immobilization efficiency, thermal treatment of the PEG melt on PFPA-functionalized surfaces resulted in excellent film quality, giving, for example, a grafting density of 9.2×10(-4)Å(-2) and an average distance between grafted chains of 33Å for PEG 20,000. The anti-fouling property of the films was evaluated by fluorescence microscopy and surface plasmon resonance imaging (SPRi). Low protein adsorption was observed on thermally-immobilized PEG whereas the photoimmobilized PEG showed increased protein adsorption. In addition, protein arrays were created using polystyrene (PS) and PEG based on the differential protein adsorption of the two polymers.

A Highly Sensitive and Selective Surface-Enhanced Nanobiosensor

Nanosphere lithography (NSL) derived triangular Ag nanoparticles were used to create an extremely sensitive and specific optical biological and chemical nanosensor. Using simple UV-vis spectroscopy, biotinylated surface-confined Ag nanoparticles were used to detect streptavidin down to one picomolar concentrations. The system was tested for nonspecific binding interactions with bovine serum albumin and was found to display virtually no adverse results. The extremely sensitive and selective response of the Ag nanoparticle sensor indicates an exciting use for biological and chemical sensing.

Resolution enhancement in surface plasmon resonance sensor based on waveguide coupled mode by combining a bimetallic approach

In this study, we present and demonstrate a new route to a great enhancement in resolution of surface plasmon resonance sensors. Basically, our approach combines a waveguide coupled plasmonic mode and a kind of Au/Ag bimetallic enhancement concept. Theoretical modeling was carried out by solving Fresnel equations for the multilayer stack of prism/Ag inner-metal layer/dielectric waveguide/Au outer-metal layer. The inner Ag layer couples incident light to a guided wave and makes more fields effectively concentrated on the outer Au surface. A substantial enhancement in resolution was experimentally verified for the model stack using a ZnS-SiO₂ waveguide layer.

Reverse engineering the yeast RNR1 transcriptional control system

Transcription is controlled by multi-protein complexes binding to short non-coding regions of genomic DNA. These complexes interact combinatorially. A major goal of modern biology is to provide simple models that predict this complex behavior. The yeast gene RNR1 is transcribed periodically during the cell cycle. Here, we present a pilot study to demonstrate a new method of deciphering the logic behind transcriptional regulation. We took regular samples from cell cycle synchronized cultures of Saccharomyces cerevisiae and extracted nuclear protein. We tested these samples to measure the amount of protein that bound to seven different 16 base pair sequences of DNA that have been previously identified as protein binding locations in the promoter of the RNR1 gene. These tests were performed using surface plasmon resonance. We found that the surface plasmon resonance signals showed significant variation throughout the cell cycle. We correlated the protein binding data with previously published mRNA expression data and interpreted this to show that transcription requires protein bound to a particular site and either five different sites or one additional sites. We conclude that this demonstrates the feasibility of this approach to decipher the combinatorial logic of transcription.

Recent advancements in optical DNA biosensors: exploiting the plasmonic effects of metal nanoparticles

The emerging field of plasmonics, the study of electromagnetic responses of metal nanostructures, has revealed many novel signal enhancing phenomena. As applied to the development of label-free optical DNA biosensors, it is now well established that plasmon-based surface enhanced spectroscopies on nanostructured metal surfaces or metal nanoparticles can markedly improve the sensitivity of optical biosensors, with some showing great promise for single molecule detection. In this review, we first summarize the basic concepts of plasmonics in metal nanostructures, as well as the characteristic optical phenomena to which plasmons give rise. We will then describe recent advances in optical DNA biosensing systems enabled by metal nanoparticle-derived plasmonic effects, including the use of surface enhanced Raman scattering (SERS), colorimetric methods, "scanometric" processes, and metal-enhanced fluorescence (MEF).

Detection of gastric carcinoma-associated antigen MG7-Ag in human sera using surface plasmon resonance sensor

BACKGROUND

MG7-Ag is a kind of gastric cancer-specific tumor-associated antigen and has been investigated to serve as a marker of gastric cancer for early diagnosis.

METHODS

Surface plasmon resonance (SPR) sensor was used for the detection of MG7-Ag in the sera of gastric cancer patients to develop an innovative, simple and rapid assay method for early diagnosis. The specific monoclonal MG7 antibodies were used as capture and detection receptors which were immobilized on the surface of SPR sensor chips for MG7-Ag identification in the human sera. The measurements include 9 cases of gastric cancer patients and 2 cases of healthy blood donors and a MKN45 cancer cell lysate solution sample for positive control.

RESULTS

The binding of MG7-Ag onto the sensor surface was observed from SPR spectra. The sera of most gastric cancer patients revealed much higher expression level of MG7-Ag than healthy human sera did in SPR measurement.

CONCLUSION

The initial results demonstrate that the SPR biosensor has the potential for its application in the early diagnosis of gastric cancer. However, more tests need to be done to confirm the detection limitation and the criterion for cancer risk evaluation in early diagnosis.

Application of AFM and optical biosensor for investigation of complexes formed in P450-containing monooxygenase systems

Atomic force microscopy (AFM) allows to visualize and count the individual protein molecules and their complexes within multiprotein systems. On the other hand, optical biosensor (OB) provides information on complex formation kinetics as well as complex lifetime (τ(LT)) and affinity. Comparison of complex lifetime τ(LT) with the time required for enzyme's catalytic cycle (τ(cat)) enables to characterize productive complexes and distinguish them from non-productive ones. Both these approaches were applied for the analysis of the three cytochrome P450-containing monooxygenase systems: cytochrome P450 101, cytochrome P450 11A1 and cytochrome P450 2B4. By using AFM, the formation of binary and ternary protein complexes was registered in all the three systems. OB analysis enabled to kinetically characterize these binary and ternary complexes. It was shown that the binary complexes putidaredoxin reductase (PdR)/putidaredoxin (Pd) and Pd/cytochrome P450 101 (P450 101) formed within the P450 101 system and, also, the binary complexes adrenodoxin reductase (AdR)/adrenodoxin (Ad) and Ad/cytochrome P450 11A1 (P450 11A1) formed within the P450 11A1 system are non-productive (deadlock). At the same time, the ternary PdR/Pd/P450 101 and AdR/Ad/P450 11A1 complexes proved to be productive. The binary cytochrome P450 reductase (Fp)/cytochrome P450 2B4 (2B4) complexes and the ternary Fp/2B4/cytochrome b5 (b5) complexes formed within P450 2B4 system were productive.

High throughput methods applied in biomaterial development and discovery

The high throughput discovery of new bio materials can be achieved by rapidly screening many different materials synthesised by a combinatorial approach to identify the optimal composition that fulfils a particular biomedical application. Here we review the literature in this area and conclude that for polymers this process is best achieved in a microarray format, which enable thousands of cell-material interactions to be monitored on a single chip. Polymer microarrays can be formed by printing pre-synthesised polymers or by printing monomers onto the chip where on-slide polymerisation is initiated. The surface properties of the material can be analysed and correlated to the biological performance using high throughput surface analysis, including time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS) and water contact angle (WCA) measurements. This approach enables the surface properties responsible for the success of a material to be understood, which in turn provides the foundations of future material design. The high throughput discovery of materials using polymer microarrays has been explored for many cell-based applications including the isolation of specific cells from heterogeneous populations, the attachment and differentiation of stem cells and the controlled transfection of cells. Further development of polymerisation techniques and high throughput biological assays amenable to the polymer microarray format will broaden the combinatorial space and biological phenomenon that polymer microarrays can explore, and increase their efficacy. This will, in turn, facilitate the discovery of optimised polymeric materials for many biomaterial applications.

Plasmonic phenomena in indium tin oxide and ITO-Au hybrid films

The observation of surface-plasmon resonances in indium tin oxide (ITO) thin films is complemented with the effects of hybrid ITO/Au conducting layers where charge densities can be tuned. Where carrier densities are similar (ITO and nanoparticle Au), the plasmonic behavior is that of a monolithic ITO thin film. Where the carrier density of one layer is much greater than that of the other (ITO and Au metal), boundary conditions lead to cancelation of the surface plasmon. In the latter case a capacitivelike plasmon resonance is observed for sufficiently thin films.

Surface plasmon resonance imaging of polymer microarrays to study protein-polymer interactions in high throughput

Polymer microarrays provide a high-throughput format in which to assess biointerfacial interactions. This endeavor greatly assists with the development of advanced biomaterials. In order to increase the scope of this platform technology, the development of analytical tools that are compatible with the microarray format and are capable of analyzing biomolecular interactions in high throughput is needed. Here, we show that surface plasmon resonance imaging (SPRi) is such a tool. SPRi enables spatially resolved, surface sensitive, label free, real-time analysis of multiple surface-biomolecular interactions in parallel. In order to demonstrate this, we first printed phenylazide-modified polymers onto a slide coated with a low fouling base polymer. UV irradiation of the slide resulted in the cross-linking of the printed polymer spots to the surface. SPRi was then employed to study the adsorption and desorption of bovine serum albumin, collagen, and fibronectin to these adhesive microarray spots. The spots were also incubated with an adherent cell line, enabling insight into the underlying mechanisms of cell attachment to the polymers studied. For the system analyzed here, electrostatic interactions were shown to dominate cell attachment.

Chemical synthesis of novel plasmonic nanoparticles

Under the irradiation of light, the free electrons in a plasmonic nanoparticle are driven by the alternating electric field to collectively oscillate at a resonant frequency in a phenomenon known as surface plasmon resonance. Both calculations and measurements have shown that the frequency and amplitude of the resonance are sensitive to particle shape, which determines how the free electrons are polarized and distributed on the surface. As a result, controlling the shape of a plasmonic nanoparticle represents the most powerful means of tailoring and fine-tuning its optical resonance properties. In a solution-phase synthesis, the shape displayed by a nanoparticle is determined by the crystalline structure of the initial seed produced and the interaction of different seed facets with capping agents. Using polyol synthesis as a typical example, we illustrate how oxidative etching and kinetic control can be employed to manipulate the shapes and optical responses of plasmonic nanoparticles made of either Ag or Pd. We conclude by highlighting a few fundamental studies and applications enabled by plasmonic nanoparticles having well-defined and controllable shapes.

Nanoscale labels: nanoparticles and liposomes in the development of high-performance biosensors

Technology for the detection of biological species has generated considerable interest in a variety of fields including healthcare, defense, food and environmental monitoring. In a biosensor, labeled specific binding partners are used to emit a detectable signal. Owing to their unique properties, nanomaterials have been proposed as a novel label category and have led to the development of new assays and new transduction mechanisms. In this article, the role of three major types of nanoscale labels (metallic, semiconductor and liposome nanoparticles) in the development of a new generation of optical, electrochemical or gravimetric biosensors will be presented. The underlying transduction principles will be briefly explained and assay strategies relying on the use of these ‘nanolabels’ will be described. The contribution to increased assay performance and sensitivity will be highlighted. Approaches towards simple, cost efficient and sensitive assays are essential to meet the demands of a growing number of applications.

Using spectroscopic ellipsometry to characterize and apply the optical constants of hollow gold nanoparticles

In this paper, we report the optical constants (refractive index, extinction coefficient) of self-assembled hollow gold nanoparticle (HGN) monolayers determined through spectroscopic ellipsometry (SE). We prepared a series of HGNs exhibiting various morphologies and surface plasmon resonance (SPR) properties. The extinction coefficient (k) curves of the HGN monolayers exhibited strong SPR peaks located at wavelengths that followed similar trends to those of the SPR positions of the HGNs in solution. The refractive index (n) curves exhibited an abnormal dispersion that was due to the strong SPR extinction. The values of Deltan and kmax both correlated linearly with the particle number densities. From a comparison of the optical constant values of HGNs with those of solid Au nanoparticles (NPs), we used SE measurements to demonstrate a highly sensitive Si-based chemical sensor. HGNs display a slightly lower value of k at the SPR peak but a much higher sensitivity to changes in the surrounding medium than do solid Au NPs.

Multiplexed plasmonic sensing based on small-dimension nanohole arrays and intensity interrogation

We performed multiplexed sensing on nanohole array devices to simultaneously obtain information on molecular absorption, scattering, and refractive-index change, which were distinguished by using different array structures with distinct optical behavior. Up to 25 arrays were fabricated within a 65 microm x 50 microm area to provide real-time information of the local surface environment. The performance of multiplexed sensing was examined by flowing NaCl, Coomassie blue, bovine serum albumin, and liposome solutions that exhibit different visible light absorption/scattering properties and different refractive indices. Experimental artifacts from light source fluctuation, sample injections, and light scattering induced by aggregates in solutions were detected by monitoring superwavelength holes or nanohole arrays with different periodicity and hole diameters.

Equilibrium electrostatics of responsive polyelectrolyte monolayers

The physical behavior of polyelectrolytes at solid-liquid interfaces presents challenges both in measurement and in interpretation. An informative, yet often overlooked, property that characterizes the equilibrium organization of these systems is their membrane or rest potential. Here a general classification scheme is presented of the relationship between the rest potential and structural response of polyelectrolyte films to salt concentration. A numerical lattice theory, adapted from the polymer community, is used to analyze the rest potential response of end-tethered polyelectrolyte layers in which electrostatics and short-range contact interactions conspire to bring about different structural states. As an experimental quantity the rest potential is a readily accessible, nonperturbing metric of the equilibrium structure of a polyelectrolyte layer. A first set of measurements is reported on monolayers of end-tethered, single-stranded DNA in monovalent (NaCl) and divalent (MgCl(2)) counterion environments. Intriguingly, in NaCl electrolyte at least two different mechanisms appear by which the DNA layers can structurally relax in response to changing salt conditions. In MgCl(2) the layers appear to collapse. The possible molecular mechanisms behind these behaviors are discussed. These studies provide insight into phenomena more generally underlying polyelectrolyte applications in the chemical, environmental, and biotechnological fields.

Nanohole arrays of mixed designs and microwriting for simultaneous and multiple protein binding studies

We demonstrate using nanohole arrays of mixed designs and a microwriting process based on dip-pen nanolithography to monitor multiple, different protein binding events simultaneously in real-time based on the intensity of Extraordinary Optical Transmission of nanohole arrays. The microwriting process and small footprint of the individual nanohole arrays enabled us to observe different binding events located only 16 microm apart, achieving high spatial resolution. We also present a novel concept that incorporates nanohole arrays of different designs to improve confidence and accuracy of binding studies. For proof of concept, two types of nanohole arrays, designed to exhibit opposite responses to protein bindings, were fabricated on one transducer. Initial studies indicate that the mixed designs could help to screen out artifacts such as protein intrinsic signals, providing improved accuracy of binding interpretation.

Extraordinary transmittance in three dimensional crater, pyramid, and hole-array structures prepared through reversal imprinting of metal films

We used a reversal imprinting-in-metal (RIM) process to fabricate various three-dimensional (3D) metal structures under low pressure. Molds featuring different shapes were used to pattern various subwavelength metal structures, including pyramidal, hole-array, and crater-like structures. Refractive index matching and cavity effects both enhanced the degree of transmission of these structured metal films. The crater-like structure appears to be a promising material because of the unique properties imparted by the elongated and gradually tapering spacing of its cavities. From both near-field simulations and experimentally obtained optical spectra, we found that the cavity effect in the crater-like structure led to significantly enhanced transmission of the optical intensity. Thus, this RIM process allows the ready fabrication of various two- and three-dimensional metallic structures for use in surface plasmon-based devices.

Exploitation of multilayer coatings for infrared surface plasmon resonance fiber sensors

We demonstrate surface plasmon resonance (SPR) fiber devices based upon ultraviolet inscription of a grating-type structure into both single-layered and multilayered thin films deposited on the flat side of a lapped D-shaped fiber. The single-layered devices were fabricated from germanium, while the multilayered ones comprised layers of germanium, silica, and silver. Some of the devices operated in air with high coupling efficiency in excess of 40 dB and an estimated index sensitivity of Delta lambda/Delta n=90 nm from 1 to 1.15 index range, while others provided an index sensitivity of Delta lambda/Delta n=6790 nm for refractive indices from 1.33 to 1.37.

Virus-based chemical and biological sensing

Viruses have recently proven useful for the detection of target analytes such as explosives, proteins, bacteria, viruses, spores, and toxins with high selectivity and sensitivity. Bacteriophages (often shortened to phages), viruses that specifically infect bacteria, are currently the most studied viruses, mainly because target-specific nonlytic phages (and the peptides and proteins carried by them) can be identified by using the well-established phage display technique, and lytic phages can specifically break bacteria to release cell-specific marker molecules such as enzymes that can be assayed. In addition, phages have good chemical and thermal stability, and can be conjugated with nanomaterials and immobilized on a transducer surface in an analytical device. This Review focuses on progress made in the use of phages in chemical and biological sensors in combination with traditional analytical techniques. Recent progress in the use of virus-nanomaterial composites and other viruses in sensing applications is also highlighted.

High-throughput SPR biosensor

Surface plasmon resonance (SPR) imaging technique is label free, real-time, and high-throughput analysis method for interaction studies with array format. The application of SPR imaging for the small molecule arrays, which were fabricated by photoaffinity crosslinking, can be the first screening step for reverse chemical genomics. The fabrication process of sugar array and sugar-lectin interaction study was demonstrated. The protocol of array fabrication did not require any chemical modifications of sugar chains for immobilizations. The biotinylated sugars were used to investigate signal ratios between lectin and antistreptavidin antibody binding. And it seemed that signal normalization could be achieved, even though the accurate densities of immobilized sugars were unclear.

High-resolution surface-plasmon resonance real-time imaging

We use surface-plasmon resonance in a silver film to obtain high-resolution real-time images of a transparent dielectric sample in contact with it. A new aspect of the work was the use of radially polarized illumination from a LED at 530 nm to obtain speckle-free images with high spatial resolution along all orientations. The sensitivity to refractive index changes in the sample is estimated to be better than 10(-3), and the modulation transfer function out to spatial frequency 1 microm(-1) was measured.

Optimization of immobilized bacterial disaccharides for surface plasmon resonance imaging measurements of antibody binding

The interactions between proteins and immobilized carbohydrates are crucial to biological events such as cell signaling and immune response. The modification of surfaces with carbohydrates to create sensing platforms provides a pathway to study these interactions in a laboratory setting. In this work, a family of structurally related Salmonella disaccharide epitopes is immobilized on thin gold films in an array format to probe antibody binding with surface plasmon resonance (SPR) imaging. The disaccharides are modified with an alkyl thiol linker for facile immobilization to gold. Small differences in the stereochemistry of the immobilized, modified disaccharides are shown to greatly influence the binding of a monoclonal antibody. Specifically, binding is only observed to an immobilized abequose dideoxyhexose relative to a tyvelose or a paratose analogue. However, both the amount and relative strength of bound antibody depends on the distribution of disaccharide moieties in a mixed monolayer of the epitope and a nonbinding diluent molecule. We thoroughly characterize the mixed monolayers with a variety of techniques to understand the optimal density and distribution of the disaccharide for antibody capture. This work reinforces the importance of controlling the density of ligands at the interface for optimized surface based bioassays.

Surface plasmon resonance imaging of cell-substrate contacts with radially polarized beams

We demonstrate the proof-of-concept for surface plasmon resonance sensing and imaging via a virtual probe at the cell-substrate interface of a biological cell in aqueous media. The technique is based on the optical excitation by focused radially polarized beams of localized surface plasmons, which forms a virtual probe on the metal substrate. The intensity distribution at the back focal plane of the objective lens enables quantitative measurements to be made of the cell-substrate contact. The acquired data is then visualized in the form of a local refractive index map.

Surface plasmon resonance spectro-imaging sensor for biomolecular surface interaction characterization

Surface plasmon resonance (SPR) techniques have become, over the last ten years, powerful tools to study biomolecular surface interaction kinetics in real-time without any use of labels. The highest resolution is currently obtained using spectroscopic SPR systems through the measurement of the complete surface plasmon resonance curve in angular or spectral configuration. But, these systems are limited to a few independent channels (<10). In order to expand their capability to an array format, SPR sensors have also been developed in an imaging mode, allowing parallel monitoring of hundreds of sensing spots onto a camera. However, such sensors rely on the intensity variation measurement at a single position of the resonance spectrum, hence resulting in smaller resolution. We present in this work a SPR spectro-imaging system which aims at keeping the advantage of a mono-channel SPR sensor based on the full resonance curve measurement while introducing an additional spatial dimension (linear multi-spot array). The system is based on the illumination of a biochip through a vertical slit (y-dimension) by a white light source. The reflected light spectrum obtained through a diffracting grating is then imaged on the x-dimension of the camera. The complete spectral resonance curve of a full column of sensing spots can be monitored in parallel and in real-time. We demonstrate that data processing is key to reduce the noise and to improve the resolution. We report on the detection of signals with resolution comparable to the one obtained with a classical SPR mono-channel spectroscopic sensor (3.5 x 10(-7) Refractive Index Unit), gaining an order of magnitude compared to SPR imaging sensors. Eventually, we show that short base DNA-DNA hybridizations with concentrations as low as 100 pM can be detected and discriminated in a few tens of minutes following injection by the SPR spectro-imaging system.

Biotin-containing phospholipid vesicle layer formed on self-assembled monolayer of a saccharide-terminated alkyl disulfide for surface plasmon resonance biosensing

We describe a technique to form a biotin-containing phospholipid vesicle layer on a self-assembled monolayer (SAM) deposited on a gold surface to immobilize biotinylated receptor proteins for a surface plasmon resonance (SPR) biosensor. The adsorption of vesicle of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) was examined by SPR on the SAMs of dithiobis(1-deoxy-glucitol-1-carbamoyl pentane) (DDGP), 11-mercaptoundecanoic acid, 11-mercaptoundecanol, 11-amino-1-undecanethiol, and 12-mercaptododecane, and it was found that the DOPC vesicle rapidly adsorbed on the DDGP SAM to achieve the highest coverage of the surface. By quartz crystal microbalance with dissipation monitoring (QCM-D), the DOPC layer formed on the DDGP SAM was shown to be a vesicle layer, in which intact DOPC vesicles physisorbed on the SAM surface. To immobilize a biotinylated receptor protein, one of three biotinylated phospholipids, 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(biotinyl) (biotin-DOPE), N-((6-(biotinoyl)amino)hexanoyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (biotin-X-DHPE) and N-(biotinoyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (biotin-DHPE), was mixed with DOPC to form a biotin-containing vesicle layer on the DDGP SAM. A comparative binding study of NeutrAvidin and the biotin-containing vesicle layers showed that the use of biotin-X-DHPE achieved the most rapid immobilization of NeutrAvidin on the vesicle layer at the highest surface density. Furthermore, biotinylated protein A, as a receptor protein, could be immobilized through NeutrAvidin on the vesicle layer containing DOPC and biotin-X-DHPE, and its reaction with immunoglobulin G, as an analyte, was successfully observed by SPR. The results demonstrate that the biotin-containing vesicle layer on the DDGP SAM must be a useful component for SPR biosensor surfaces.

Excitation and detection of surface-plasmon polariton waves on Cu(111) with gratings of rare gas monolayers

We excited surface-plasmon polariton waves (SPPWs) on Cu(111) by coupling optical beams with adsorbed xenon gratings. The SPPWs's excitation causes a resonancelike dip in the angle-resolved reflectivity difference measurement. From the resonance we determined optical constants epsilonCu(633 nm)=-9.53+i0.142 and epsilonCu(780 nm)=-13.44+i0.18. The grating-coupled SPPWs can be used to study mass transport on thin films.

Label-free biosensors based on optically responsive nanocomposite layers: sensitivity and dynamic range

Core-shell nanoparticle layers have proven to be a promising tool for the label-free detection of binding events. Upon reflection of white light, they exhibit pronounced extinction peaks in the UV/vis and NIR regime of the electromagnetic spectrum, which shift to higher wavelengths when molecules are adsorbed. Beside drastic simplification of the instrumentation and related reduction in cost, a significantly stronger response toward alkanethiol adsorption has been observed in previous experiments than in conventional surface plasmon resonance (SPR). However, as the amount of molecules deposited onto the nanoparticle films was unknown, no quantitative relationship could be established between the measured wavelength shifts and the surface mass density of the adsorbate. In order to facilitate quantitative molecule detection, self-assembled monolayers (SAMs) of simple and ethylene glycol (EG) terminated alkanethiols with various chain lengths were prepared on the nanoparticle-coated substrates. The measured red-shift of the extinction spectrum upon molecule adsorption was related to the amount of adsorbate as determined by X-ray photoelectron spectroscopy (XPS). For the whole range of film thicknesses studied, a linear relationship is found yielding a sensitivity factor of 0.027 nm/(ng/cm (2)). As proven by enzyme-linked immunosorbent assay (ELISA), such determined sensitivity factor can also be used to correctly predict the amount of surface-bound protein in immunoreactions from the measured wavelength shifts. It is concluded that the decay length of the evanescent electric field associated with the nanoparticle sensors is more than 100 nm and, thus, significantly larger than that observed for localized surface plasmons excited in small isolated metal clusters.

Discovery and verification of functional single nucleotide polymorphisms in regulatory genomic regions: current and developing technologies

The most common form of genetic variation, single nucleotide polymorphisms or SNPs, can affect the way an individual responds to the environment and modify disease risk. Although most of the millions of SNPs have little or no effect on gene regulation and protein activity, there are many circumstances where base changes can have deleterious effects. Non-synonymous SNPs that result in amino acid changes in proteins have been studied because of their obvious impact on protein activity. It is well known that SNPs within regulatory regions of the genome can result in disregulation of gene transcription. However, the impact of SNPs located in putative regulatory regions, or rSNPs, is harder to predict for two primary reasons. First, the mechanistic roles of non-coding genomic sequence remain poorly defined. Second, experimental validation of the functional consequences of rSNPs is often slow and laborious. In this review, we summarize traditional and novel methodologies for candidate rSNPs selection, in particular in silico techniques that aid in candidate rSNP selection. Additionally we will discuss molecular biological techniques that assess the impact of rSNPs on binding of regulatory machinery, as well as functional consequences on transcription. Standard techniques such as EMSA and luciferase reporter constructs are still widely used to assess effects of rSNPs on binding and gene transcription; however, these protocols are often bottlenecks in the discovery process. Therefore, we highlight novel and developing high-throughput protocols that promise to aid in shortening the process of rSNP validation. Given the large amount of genomic information generated from a multitude of re-sequencing and genome-wide SNP array efforts, future focus should be to develop validation techniques that will allow greater understanding of the impact these polymorphisms have on human health and disease.